FIG. 1 illustrates a conventional flip chip LED die 10 mounted on a submount 12. In a flip-chip, both the n and p electrodes are formed on the bottom of the LED die.
The LED die 10 is formed of semiconductor epitaxial layers, including an n-layer 14, an active layer 15, and a p-layer 16, grown on a growth substrate, such as a sapphire substrate. The growth substrate has been removed in FIG. 1 by laser lift-off, etching, grinding, or by other techniques. In one example, the epitaxial layers are GaN based, and the active layer 15 emits blue light.
A pre-fabricated phosphor tile 17 is affixed over the top surface of the LED die 10 by silicone or other transparent adhesive (not shown). For white light, the tile 17 may be formed of a YAG phosphor, which emits yellow light. Some of the blue LED light leaks through the tile 17 and combines with the yellow light. A red phosphor layer may be added to create a warmer white light.
A metal electrode 18 electrically contacts the p-layer 16, and a metal electrode 20 electrically contacts the n-layer 14. In one example, the electrodes 18 and 20 are gold pads that are ultrasonically welded to anode and cathode metal pads 22 and 23 on a ceramic submount 12. The submount 12 has conductive vias 24 leading to bottom metal pads 26 and 28 for bonding to a printed circuit board.
U.S. Pat. No. 8,536,608, assigned to the present assignee and incorporated herein by reference, describes the use of a reflective ring 29 (e.g., R>90%) affixed to the surface of the submount 12 and surrounding the LED die 10. The ring 29 is formed of a reflective material, such as a sputtered Al or Ag layer. The ring 29 reflects light generated downward by the phosphor tile 17 and the active layer 15 (e.g., light rays 32 and 33). Submounts are typically ceramic, silicon, or other light absorbing material, so the ring 29 reduces the absorption of light by the package to increase efficiency.
The LED die 10, phosphor tile 17, and ring 29 are then encapsulated by a molded, transparent hemispherical lens 34, such as formed of silicone, to protect the LED die 10 and increase light extraction efficiency. The index of refraction of the lens 34 (e.g., n=1.5) is typically between the index of air and the index of the various LED layers.
Although the reflectivity of the ring 29 is relatively high, it is not 100%. Therefore, there is still some light absorption by the ring 29. Further, the outer edge of the ring 29 material must end prior to the edge of the lens 34 in order to ensure a good hermetic seal between the lens 34 and the submount surface. Therefore, there is some light scattering and absorption between the edge of the ring 29 and the edge of the lens 34. Further, forming a metal ring is relatively complex and adds cost to the package.
What is needed is a technique to reflect even more downward light from the LED die and the phosphor tile.